1. Field of the Invention
The present invention relates to a protection circuit for an amplification device.
2. Background of the Invention
Typically, amplifiers use a constant protection level or “threshold”. When the threshold is exceeded a protection circuit is invoked, which shuts down the amplifier or otherwise causes it to protect itself from permanent damage. The protection threshold needs to be placed outside the extreme limit of operation of the amplifier to prevent its unintentional activation during normal usage.
The problem with currently available protection techniques is that fault conditions also occur in scenarios where the protection threshold is not exceeded. One such scenario is when the system is driving a short, during low output levels. Under conditions when the output voltage is high, the modulation level of the amplifier is also high. In this case, the system runs efficiently, so a large input current being carried through the system is not a problem. Under conditions when the output voltage is low, the modulation level of the amplifier is also low. In this case, the system runs inefficiently, so a large current being carried through the system may be a problem.
A short circuit condition, when it occurs at a time when the output voltage is high, is typically not a problem because the system can handle a high current surge when it is running efficiently and it will shut down, because in this condition, prior art current protection thresholds are designed to shut the amplifier off.
However, a short circuit condition, when it occurs at a time when the output voltage is low, may be a problem. When the output voltage is low, the output modulation level is also low. This causes the system to run inefficiently. Moreover, during this condition the current will not be high enough to reach prior art protection thresholds, so the system will not shut down. Since the system is running inefficiently in the latter short circuit condition and will not shut down, and at the same time sourcing a high current, the system components overheat, and/or become damaged.
In many of these cases the eventual destruction of the amplifier will occur due to excess power dissipation in the amplifier output section. For example, if the speaker output is shorted during a low output level or low volume, the amplifier might not reach its protection level and never invoke the protection circuit.
Therefore, the amplifier will continue to operate in this fault condition, which may cause excessive power dissipation within the amplifier and its failure, despite having a protection circuit with a protection threshold in place.